The present invention relates to a hydrogen generating process and apparatus and, more particularly, to a water gas shift reactor which is suitable for use as a hydrogen purification system or as an electric power generation system when used in conjunction with a fuel cell.
Fuel cells are chemical power sources in which electrical power is generated in a chemical reaction. The most common fuel cell is based on the chemical reaction between a reducing agent such as hydrogen and an oxidizing agent such as oxygen. The consumption of these agents is proportional to the power load. Because hydrogen is difficult to store and distribute and because hydrogen has a low volumetric energy density compared to fuels such as gasoline, hydrogen for use in fuel cells will have to be produced at a point near the fuel cell, rather than be produced in a centralized refining facility and distributed like gasoline.
Hydrogen is widely produced for chemical and industrial purposes by converting materials such as hydrocarbons and methanol in a reforming process to produce a synthesis gas.
Synthesis gas is the name generally given to a gaseous mixture principally comprising carbon monoxide and hydrogen, but also possibly containing carbon dioxide and minor amounts of methane and nitrogen. It is used, or is potentially useful, as feedstock in a variety of large-scale chemical processes, for example: the production of methanol, the production of gasoline boiling range hydrocarbons by the Fischer-Tropsch process and the production of ammonia.
Processes for the production of synthesis gas are well known and generally comprise steam reforming, auto-thermal reforming, non-catalytic partial oxidation of light hydrocarbons or non-catalytic partial oxidation of any hydrocarbons. Of these methods, steam reforming is generally used to produce synthesis gas for conversion into ammonia or methanol. In such a process, molecules of hydrocarbons are broken down to produce a hydrogen-rich gas stream.
Modifications of the simple steam reforming processes have been proposed. In particular, there have been suggestions for improving the energy efficiency of such processes in which the heat available from a secondary reforming step is utilized for other purposes within the synthesis gas production process. For example, processes are described in U.S. Pat. No. 4,479,925 in which heat from a secondary reformer is used to provide heat to a primary reformer.
The reforming reaction is expressed by the following formula:
CH4+2H2Oxe2x86x924H2+CO2
where the reaction in the reformer and the reaction in the shift converter are respectively expressed by the following formulae (1) and (2)
CH4+H2Oxe2x86x92CO+3H2
CO+H2Oxe2x86x92H2+CO2
In the conventional hydrogen generating apparatus, an inert gas heated in a reformer is made to flow through a process flow path so as to raise temperatures of the shift converter and the heat exchangers which are downstream from the reformer.
U.S. Pat. No. 5,110,559 discloses an apparatus for hydrogen generation which includes a reformer and a shift converter each incorporating a catalyst wherein, during the start-up of the apparatus, reformer combustion gas is introduced to a shift converter jacket surrounding the shift converter catalyst to heat the shift converter to provide a start-up or temperature rise of the reformer system.
U.S. Pat. No. 4,925,456 discloses a process and an apparatus for the production of synthesis gas which employs a plurality of double pipe heat exchangers for primary reforming in a combined primary and secondary reforming process. The primary reforming zone comprises at least one double-pipe heat exchanger-reactor and the primary reforming catalyst is positioned either in the central core or in the annulus thereof. The invention is further characterized in that the secondary reformer effluent is passed through which ever of the central core or the annulus is not containing the primary reforming catalyst countercurrently to the hydrocarbon-containing gas stream.
U.S. Pat. No. 5,181,937 discloses a system for steam reforming of hydrocarbons into a hydrogen rich gas which comprises a convective reformer device. The convective reformer device comprises an outer shell enclosure for conveying a heating fluid uniformly to and from a core assembly within the outer shell. The core assembly consists of a multiplicity of tubular conducts containing a solid catalyst for contacting a feed mixture open to the path of the feed mixture flow such that the path of the feed mixture flow is separated from the heating fluid flow in the outer shell. In the process, an auto-thermal reformer fully reforms the partially reformed (primary reformer) effluent from the core assembly and supplies heat to the core assembly by passing the fully reformed effluent through the outer shell of the convective reforming device.
Fuel cells are chemical power sources in which electrical power is generated in a chemical reaction. The most common fuel cell is based on the chemical reaction between a reducing agent such as hydrogen and an oxidizing agent such as oxygen. The consumption of these agents is proportional to the power load. Because hydrogen is difficult to store and distribute and because hydrogen has a low volumetric energy density compared to fuels such as gasoline, hydrogen for use in fuel cells will have to be produced at a point near the fuel cell, rather than be produced in a centralized refining facility and distributed like gasoline. Polymers with high protonic conductivities are useful as proton exchange membranes (PEM""s) in fuel cells. Among the earliest PEM""s were sulfonated, crosslinked polystyrenes. More recently sulfonated fluorocarbon polymers have been considered. Such PEM""s are described in an article entitled, xe2x80x9cNew Hydrocarbon Proton Exchange Membranes Based on Sulfonated Styrene-Ethylene/Butylene-Styrene Triblock Copolymersxe2x80x9d, by G. E. Wnek, J. N. Rider, J. M. Serpico, A. Einset, S. G. Ehrenberg, and L. Raboin presented in the Electrochemical Society Proceedings (1995), Volume 95-23, pages 247 to 251.
The above processes generally relate to very large industrial facilities and the techniques for integrating the steps of converting the hydrocarbon or alcohol feedstream may not be useful in compact, small-scale hydrogen-producing units to power a transportation vehicle or to supply power to a single residence. One of the problems of large hydrogen facilities is the problem of methane slippage in steam reforming reactors. Methane slippage is a term used to describe a reduction in the methane conversion across the reforming reactor. Generally, the equilibrium conversion of methane to carbon oxides and hydrogen that is achieved in the reforming reactor increases with temperature. Consequently, a reduction in the reactor outlet temperature corresponds to a lower conversion of methane, or a methane slippage. Methane slippage reduces the overall production of hydrogen and hence the efficiency of the process. Methane slippage can create problems in downstream equipment such as in an oxidation step used to remove trace amounts of carbon monoxide from the hydrogen stream before passing the hydrogen stream to the fuel cell.
It is the objective of this invention to provide a compact apparatus for generating hydrogen from available fuels such as natural gas, hydrocarbons, and alcohols for use in a fuel cell to generate electric power.
It is an objective of this invention to provide an integrated fuel cell and hydrogen production system which is energy and hydrogen efficient.
The compact water gas shift apparatus of the present invention provides a simple and efficient system for enhancing the production of hydrogen from the effluent of a steam reforming process or an autothermal reforming process or a combination thereof. By disposing both the high temperature and the low temperature water gas shift reaction zones in a vertically aligned chamber and introducing the water for both cooling the feed and for the reaction as spray stream and providing dispersion zones to disperse the water spray into the effluent from the reaction zones, the feeds to the water gas shift reaction zones can be maintained at effective high and low water gas shift conditions without damaging the shift catalyst. Water gas shift catalyst can be damaged by direct contact with cold water and the use of the dispersion zones between the spray zones and the shift reaction zones solves this problem which on this small-scale unit reduces the complexity of the water gas shift reaction zones and provides an efficient means for controlling the temperature within the exothermic reaction zones. Furthermore, the cooling coil located in the low temperature shift reaction zone provides a means for controlling temperature excursions in the low temperature reaction zone when the temperature of the low temperature shift reaction zone exceeds about 70xc2x0 C. to prevent damage to temperature sensitive downstream equipment such as the PEM fuel cell membrane. Furthermore, the cooling of the shift reactors by direct contact with water spray provides the product hydrogen stream at saturated conditions which is desirable to prevent the drying out of the PEM membrane.
In one embodiment, the present invention is an apparatus for a compact water gas shift reaction zone for removing carbon monoxide from a hydrogen stream for the production of electric power from a fuel cell. The apparatus comprises a vertically aligned vessel having a top end, a bottom end opposite and which defines an interior space. The vertically aligned cylindrical vessel defines a shift inlet at the top end, and a shift outlet at the bottom end. A first water spray nozzle in communication with a first water supply conduit is located within the interior space for contacting the hydrogen stream with a first water stream. A first dispersion material is retained in the interior space and is disposed below the first water spray nozzle to define the bottom of a first water spray zone. The first dispersion material defines a first dispersion zone in fluid communication with the water spray zone to disperse the first water stream into the hydrogen stream. A first fluid permeable portion defines the bottom of a high temperature shift zone in the interior space disposed below the first dispersion zone. The high temperature shift zone is in fluid communication with the first dispersion zone. The high temperature shift zone contains a high temperature shift catalyst to produce a high temperature shift effluent stream. A second water spray zone which is in fluid communication with the high temperature shift reaction zone and is defined by a portion of the interior space is disposed below the high temperature shift zone. The second water spray zone contains a second water spray nozzle in fluid communication with a second water supply conduit to contact the high temperature shift effluent stream with a second water stream. A second dispersion zone material is retained in the interior space below the second water spray zone and is in fluid communication with the second water spray zone to define a second dispersion zone. The second dispersion zone provides for dispersing the second water spray stream into the high temperature shift effluent stream. A low temperature shift zone is in fluid communication with the second dispersion zone and is defined by a portion of the interior space below the second dispersion zone and above said shift outlet. The low temperature shift zone contains a low temperature shift catalyst to produce a water saturated hydrogen product stream. The first dispersion zone and the second dispersion zone comprise dispersion material selected from the group consisting of sand, quartz, glass, alumina, and mixtures thereof. The first fluid permeable portion comprises a screen composed of a 304, 316, or similar alloy of stainless steel. The vessel comprises a 304, 316, or similar alloy of stainless steel.
In another embodiment, the present invention is a compact water gas shift process for removing carbon monoxide from a hydrogen stream in the production of electric power from a fuel cell. The water shift process comprises passing a feed stream comprising hydrogen and carbon monoxide to a first water spray zone. In the first water spray zone the feed stream is directly contacted with a first water spray stream to cool the reactor effluent stream to effective high temperature shift conditions and to provide a high temperature shift reactor feed admixture. The high temperature shift reactor feed admixture is passed through a first dispersion zone to uniformly disperse the first water spray in the high temperature shift reactor feed stream to provide a dispersed high temperature shift reactor feedstream. The dispersed high temperature shift reactor feedstream is passed to a high temperature shift reaction zone, containing a high temperature shift catalyst, to provide a high temperature shift effluent stream. The high temperature shift effluent stream is passed to a second water spray zone wherein the high temperature shift effluent stream is directly contacted with a second water spray stream to cool the high temperature shift effluent to effective low temperature shift conditions and to provide a low temperature shift feed admixture. The low temperature shift feed admixture is passed through a second dispersion zone to uniformly disperse the second water spray in the low temperature shift feed admixture and to provide a dispersed low temperature shift admixture. The dispersed low temperature shift admixture is passed to a low temperature shift reaction zone, containing a low temperature shift catalyst, wherein the low temperature shift reactor feed stream is reacted to produce a low temperature shift effluent stream. The low temperature shift effluent stream is withdrawn at a low temperature shift effluent temperature, at saturation conditions and having a carbon monoxide concentration less than about 3000 ppm-mol.
For an electrical output of about 7 kW, the present invention required a natural gas throughput of about 2.4 normal cubic meters per hour (about 1.4 standard cubic feet per minute) thus providing an overall energy efficiency of about 30 percent.